A. Technical Field
This invention relates to the field of forward and reverse path distribution amplifiers and, in particular, to improving signal performance at the crossover between forward and reverse path frequency bands using circuitry providing a 180 degree phase reversal.
B. Description of the Relevant Art
With the emphasis recently placed on reverse path capability within, for example, cable television signal distribution systems to provide high speed modem, telecommunications and other reverse path applications, so-called loop gain in the forward and reverse sections of a cable television distribution amplifier has become an issue. Loop gain, that is, gain, within a circuit loop within an amplifier in the forward and reverse path can become a factor and cause amplifier "singing" or undesirable signal oscillations. It is important at the crossover frequency between the forward and reverse pass bands that the loop gain be less than 1. One known method for minimizing the possibility of undesirable oscillation is to filter the forward frequencies within the reverse path and to filter the reverse frequencies within the forward path. This method causes the maximum loop gain to occur at the geometric mean of the forward and reverse frequency bands (the crossover frequency) where rejection is obtained by the skirts of the suggested filters. But minimizing the loop gain at the crossover frequency places severe requirements on the filters which must roll off very rapidly to insure good crossover isolation.
Gris, U.S. Pat. Nos. 5,068,630; 5,126,702 and 5,343,158 has recognized that by providing a transformer device within a circuit having high and low pass paths, phase cancellation can occur. In particular, in Gris '630, a low-pass path through a multiple coupler includes a phase inverter transformer to enable recombination of low-pass and high pass signals without any distortion. Gris '702 shows in FIGS. 3-7 several applications of phase inverters to provide the capability of signals in each of a high- and low-pass path without distortion. Finally, Gris '158 shows the application of a phase inverter TA at column 4, line 15-25, for example, where the two signals are exactly equal, if properly balanced, and after inversion, the two signals are exactly opposite each other and cancel each other out at point 6 of FIG. 1.
While Gris et al. '158 relates to a cable television distribution amplifier, Gris relies on the component values in each leg of paths 7, 5, 6 and 7, 3, 4, 6 to be perfectly balanced and match each other which may be difficult with temperature variations and the like expected in distribution amplifiers which are used outdoors and may experience internal temperatures on the order of 95 degrees Centigrade. Moreover, Gris et al. '158 is concerned with recombining low-pass and high-pass paths at a recombination point and not with crossover compensation in a plurality of loops of a multiple output distribution amplifier. Consequently, while it is known to use phase reversal to advantage for phase cancellation, there remains a problem of improving crossover compensation at crossover frequency in a loop of a distribution amplifier having multiple outputs where the high pass/low pass frequency bands overlap.